This invention relates to a composite glass article composed of a core portion encompassed within a compressively stressed exterior portion. It is more particularly concerned with such a glass structure wherein the exterior portion is a vitreous silica-type glass.
The term "vitreous silica glass" is herein used to include any form of vitrified silica including fused quartz, fused silica, and materials such as 96% silica glass which are composed essentially of silica and have closely related physical and chemical characteristics. "Fused silica" refers to silica which is initially produced in particulate form, as by flame hydrolysis of silicon tetrachloride. Thereafter, the silica particles are formed into articles by any one of three methods: (1) collected directly in a heated chamber as a glass boule; (2) collected as a porous preform and consolidated to a non-porous glass; or (3) collected as a soot which is molded and consolidated.
A "vitreous silica-type glass" or a "fused silica-type glass" will generally possess the parent glass characteristics, but may contain additional constituents, or additives, in minor amounts to impart special radiation, transmission, or other physical characteristics.
The vitreous silicas are well known for certain physical properties which render them unique among glasses. Such properties include: excellent refractoriness which permits use at high temperatures; chemical inertness; and a low thermal coefficient of expansion (on the order of 5 to 10 .times. 10.sup.-.sup.7 /.degree.C. over the range 0.degree.-300.degree.C.) which provides high resistance to thermal shock. In spite of these exceptional attributes, use of the silica glasses has been limited by their relatively low mechanical strength and the difficulty in devising any practical means of increasing such strength.
Traditionally, glasses have been strengthened by thermal tempering; that is, sudden chilling of a glass surface from a temperature near the glass softening point, followed by slower cooling of the glass body. The degree of strengthening attainable by this procedure is dependent on the magnitude of the linear expansion coefficient of the glass. Therefore, it is not an effective procedure with silica glasses having low expansion coefficients on the order of 5-10 .times. 10.sup.-.sup.7 /.degree.C.
More recently, chemical strengthening techniques have been developed that are based on an ion exchange. These techniques are also ineffective on silica glasses since the latter contain no exchangeable ions.
It has also been long recognized that casing a high expansion glass with a lower expansion glass can yield high compressive forces and consequent high strength. The low thermal expansion coefficients of the silica glasses make them ideal candidates for skin or exterior glasses in a cased or composite structure. However, attempts to apply a silica glass over a higher expansion core glass have encountered difficulties. Whenever enough additives are introduced into a potential core glass to raise its expansion to an adequate level for high strength development, the core glass becomes too soft; that is, it has too low a viscosity at the fusing or combining temperature.